110 
THE PHYSICAL GEOGRAPHY OF THE SEA. 
east trades, ascended, and then flowed indiscriminately to the 
north or the south. 
But I saw reasons for supposing that what came to the equato- 
rial calms as the southeast trade-winds continued to the north as 
an upper current, and that what had come to the same zone as 
northeast trade-winds ascended and continued over into the south- 
ern hemisphere as an upper current, bound for the calm zone of 
Capricorn. 
And these are the principal reasons and conjectures upon which 
these suppositions were based : 
195. At the seasons of the year when the sun is evaporating 
most rapidly in the southern hemisphere, the most rain is falling 
in the northern. Therefore it is fair to suppose that much of the 
vapor which is taken up on that side of the equator is precipitated 
on this. 
The evaporating surface in the southern hemisphere is greater, 
much greater, than it is in the northern ; still, all the great rivers 
are in the northern hemisphere, the Amazon being regarded as 
common to both ; and this fact, as far as it goes, tends to corrobo- 
rate the suggestion as to the crossing of the trade-winds at the 
equatorial calms. 
196. Independently of other sources of information, my investi- 
gations also taught me to believe that the mean temperature of the 
tropical regions was higher in the northern than in the southern 
hemisphere, for they show that the difference is such as to draw 
the equatorial edge of the southeast trades far over on this side of 
the equator, and to give them force enough to keep the northeast 
trade-winds out of the southern hemisphere almost entirely. 
197. Consequently, as before stated, the southeast trade-winds 
being in contact with a more extended evaporating surface, and 
continuing in contact with it for a longer time or through a 
greater distance, they would probably arrive at the trade-wind 
place of meeting more heavily laden with moisture than the others. 
198. Taking the laws and rates of evaporation into considera- 
tion, I could find no part of the ocean of the northern hemisphere 
from which the sources of the Mississippi, the St. Lawrence, and 
the other great rivers of our hemisphere could be supplied. 
Hence, by this process of reasoning, I was induced to regard 
the extra-tropical regions of the northern hemisphere as standing 
